1. Field of the Invention
The present invention relates to the use of economically feasible diamond materials thermal management of heat-generating systems. More particularly, the present invention relates to the use of economically feasible diamond materials for thermal management of heat-generating systems including active electronic devices such as integrated circuits.
2. The Prior Art
Active electronic devices such as transistors, laser diodes, vacuum tubes, and other electron control components generate waste heat as a common and necessary result of their operation. For purposes of this application, active electronic device is meant to encompass any device that consumes electrical energy and transforms a portion of that energy to heat.
Depending on the specifics of the device, said specifics including its geometry, power dissipation, duty cycle, material, and operational requirements, the waste heat generated may constrain the device performance in an unacceptable manner or to an unacceptable degree. For example, in laser diodes, the extreme power density necessary to excite laser emission from the narrow, shallow lasing region causes heating in the device which presents particularly difficult thermal management problems. As laser diode brightness requirements increase, so too do the requirements for rejection of waste heat.
A further example may be found in microprocessors. Microprocessors are understood by those skilled in the art to comprise integrated circuits composed of multiple transistors which control the flow of electrons so as to effect computation in a useful fashion. At this writing, microprocessors are primarily fabricated using silicon both as the electronically active elements (transistors) and as the mechanical support material for those elements. This is commonly done by fabricating transistors on the surface of a silicon wafer, the transistor structures being thin compared with the bulk of the wafer thickness underlying the active device layers. It will be appreciated that although reference is made in this discussion to silicon integrated circuits such as microprocessors, this is not intended by way of limitation with regard to the material used to fabricate such devices, and other materials such as germanium, gallium arsenide, gallium aluminum arsenide, and gallium nitride are included without limitation.
Microprocessors, like other electronic devices, generate heat as a consequence of operation. Owing to the physics underlying its construction, a microprocessor generates more heat the more rapidly it is made to operate. Thus, as microprocessors are made to provide higher clock rates (an index of the speed with which the transistors in the processor are made to switch on and off, which is the basis for their computational functions), they generate greater heat which must be disposed of to maintain the microprocessor temperature within its safe operating limits.
Microprocessors present particularly difficult thermal management problems. They are composed of millions of transistors and their heat generation rates depend on their clock speeds and the particular software codes they execute, among other parameters. In addition, their temperature or thermal profiles are often nonuniform. That is, certain areas of the processor will be hotter than others, and these hot spots will limit the overall performance of the device, even though other regions are operating well within their thermal limits and could be operated faster but for the hot spot regions.
In common practice, microprocessors are mechanically constrained by a package, which consists of components that support and position the processor in space, provide electrical connections, allow for the transference of waste heat away from the device, and provide the requisite degree of mechanical integrity and isolation from environmental degradation. It will be appreciated by those skilled in the art that these requirements are often in opposition and pose difficult engineering tradeoffs to the package designer. Once a processor has been packaged, it is often affixed to a circuit board whereby it interfaces with other integrated circuits such as memory, buss management chips, or other integrated circuits that co-operate with the processor in its operation.
There are many ways to package microprocessors. For processors that have significant thermal rejection requirements, a critical element of their packaging is the component through which heat from the processor first passes on its way to the ultimate heat-dissipation modality in the system. This component, being the first element in the chain of transport of waste heat away from the processor, greatly affects the efficiency of heat transport, and is subject to a variety of conflicting requirements that, taken together, strongly constrain the choice of materials that can be used to fabricate the component. The component is often called a heat spreader, and is usually bonded to the processor by means of thermally conductive adhesive, or solder, or by use of mechanical compression or other means of affixing it in intimate contact to the processor. It should be noted that the designation “heat spreader” is a convenience for discussion of the package component and not a limiting designation.
Referring now to FIG. 1, a schematic representation shows a common configuration of a microprocessor, heat spreader, and overall package. The figure shows a “BGA (Ball Grid Array) package”, a configuration in which a microprocessor is mounted on an array of small solder spheres that primarily provide electrical contact to the microprocessor. They also lend a degree of mechanical support and thermal conduction, although the latter is relatively low because the thermal conductivity of solder alloys is poor and the total surface area available for thermal conduction is small compared to the area of the microprocessor.
The principal thermal pathway in this configuration is out the back of the microprocessor, through the heat spreader, through the package lid, and into the heat sink, from which heat is extracted, often by a moving fluid, for dispersal to the environment. Although this figure represents one particular configuration of microprocessor package, it is representative of most others in depicting the need for a high thermal conductivity pathway through which to reject heat produced during operation.
The package component, or heat spreader, immediately adjacent to the microprocessor must exhibit high thermal conductivity to facilitate the transport of heat away from the processor. It must also possess a thermal expansion coefficient that is sufficiently similar to that of the processor material to avoid exerting destructive forces on the processor which arise out of the differential thermal expansion of the processor and heat spreader. It must also be capable of supporting an intimate bond with the processor through adhesive, solder, or other means. If needed, it must provide electrical isolation without detriment to thermal transfer. These requirements are mutually conflicting, and have not heretofore been susceptible of a particularly favorable solution.
Materials to date used for microprocessor heat spreaders have been selected from metals and metal alloys such as copper/tungsten or copper/molybdenum. Certain ceramic materials, such as beryllium oxide or aluminum nitride have also been used as heat spreaders for electronic devices. These materials are characterized by having thermal conductivities in the range of approximately 1 to approximately 3 watts/centimeter/degree Centigrade (hereafter W/cm/° C.). For reference, pure copper has a thermal conductivity of approximately 4 W/cm/° C.
It will be immediately appreciated by those skilled in the art that the materials in common use for heat spreaders in microprocessor packaging exhibit lower thermal conductivity by far than a commonly available metal, copper, which would otherwise be a better choice of material for the requirement. Copper, however, has a thermal expansion coefficient much greater than that of silicon, and cannot be used in direct proximity with a silicon device due to the high probability of fracture of the silicon as a result of differential thermal expansion during operation. For comparison, silicon has a thermal expansion coefficient of approximately 4.1 parts per million per degree Centigrade, while copper, copper/tungsten alloy, beryllium oxide, and aluminum nitride are respectively approximately 17, 5.6-7, 8, and 4.5, in the same units. Aluminum nitride provides a good thermal expansion match to silicon, but its thermal conductivity is only about 1.7 W/cm/° C. Most designers therefore select copper/tungsten alloys because they are less costly than aluminum nitride and provide similar thermal conductivity.
The problem of thermal expansion becomes more severe as the size of the microprocessor or other electronic device increases. Because differential expansion forces arise over the entire extent of the interface between heat spreader and microprocessor, the larger the interface, the greater the total force exerted on the processor, and the higher the probability of fracture of the processor material. For purposes of this disclosure, we intend to cover electronic devices, including microprocessors, having a surface area in contact with the heat spreader or other first package element of one square centimeter or greater.
As a result of differential expansion forces, package designers have been forced to employ materials of inferior thermal conductivity as heat spreaders in device packages. This in turn has limited the ability to transport waste heat away from microprocessors, constraining their performance to a substantial and unacceptable degree.
It has long been known that diamond is an ideal potential candidate for heat transfer applications such as described above. Diamond, both natural and synthetic, is known in the art to exhibit thermal conductivity superior to all other known materials. Diamond manufactured by chemical vapor deposition, a modern and widely studied means of diamond synthesis, has been shown to exhibit thermal conductivity in excess of 20 W/cm/° C. At the same time, diamond's thermal expansion coefficient is approximately 1 to 1.7×10−6/° C. over the operating temperature range of silicon-based microprocessors. Finally, diamond has the highest Young's modulus, or stiffness, of any known material, excluding unidirectional modulii of certain fibers not applicable to the present invention. The combination of these properties is unique to diamond.
It has not been previously possible, however, to provide diamond material for use in such applications at a price that permits their commercial use. Integrated circuits such as microprocessors are fabricated on integrated circuit dice having areas of 1 square centimeter or larger. Use of diamond material for heat transfer from such devices having areas as large as 1 square centimeter or larger has been, at best, a laboratory curiosity because of the prohibitively high expense of providing the diamond material.
Diamond synthesis through chemical vapor deposition (CVD) is by now a well-established art. Commercial diamond deposition systems are available and products employing CVD diamond are routinely sold for commercial applications ranging from cutting tools to heat spreaders.
All diamond CVD processes to date have been characterized by very low process efficiency in terms of the amount of diamond produced in response to consumption of energy and synthesis materials. There has been a long-felt need within the CVD diamond industry to improve diamond CVD process efficiencies. This long felt need has given rise to vigorous prior but unsuccessful efforts to achieve significantly higher process efficiencies.
Diamond chemical vapor deposition is accomplished by energizing an appropriate gas mixture (most commonly containing a preponderance of hydrogen and a minor hydrocarbon constituent, the latter being the source of carbon which is deposited as diamond) in a suitable deposition reactor and providing means for the diamond precursor chemical specie(s) generated by said application of energy to encounter a surface whose temperature, chemistry, and surface preparation properties permit the nucleation and growth of diamond layers in a manner well-known to those skilled in the art.
It is thought, and there is considerable supporting and little contradicting evidence, that a key step in the formation of diamond through the majority of useful diamond CVD processes is the dissociation of ordinary molecular hydrogen gas to form atomic hydrogen. Once formed, this species effects several actions known to be required for diamond CVD to occur at useful rates: it stabilizes carbon in the sp3 (diamond) bonding configuration and forestalls the formation of undesirable sp2 (graphitic) bonds; it abstracts hydrogen from hydrocarbon precursor species and thereby makes carbon available for incorporation in the growing diamond lattice; and it selectively reacts with, and returns to the gas phase, those small graphitic domains which may from time to time appear during the diamond CVD process, thereby preventing the formation of graphite-contaminated diamond which would be of no significant utility.
While the details of surface chemistry underlying diamond CVD remain to some degree obscure, it is widely observed that the successful synthesis of diamond through most CVD methods requires production of atomic hydrogen. The economics of diamond CVD processes are essentially the economics of generating atomic hydrogen.
A few instances of diamond CVD requiring little or no atomic hydrogen are known to the art. A laser-driven diamond CVD process developed by QQC, Inc., appears not to require hydrogen in its precursor atmosphere. Similarly, diamond CVD using microwave-assisted plasmas has been accomplished under conditions of reduced hydrogen concentration. While these results suggest subtleties to diamond CVD chemistry that remain to be elucidated, they confirm the fundamental scheme in which a gas, or mixture of gases, is energized by a variety of modalities to effect diamond CVD under appropriate conditions as specifically required by the circumstances chosen by the practitioner of the art. To date, none of the low-hydrogen diamond CVD processes has demonstrated commercial utility.
In all known diamond CVD processes having commercial utility, production of atomic hydrogen is required and is the primary energy-consuming step in those processes. The costs of producing atomic hydrogen in large quantities are considerable, and include the direct cost of energy as well as the cost of equipment needed to apply that energy in a useful manner. As will be seen, the diamond CVD processes that constitute the current art are extremely inefficient, leading to very high energy and equipment costs.
The heart of diamond CVD process inefficiency lies in the short lifetime of the atomic hydrogen species and in the energy cost of creating that species. Atomic hydrogen is thermodynamically driven to recombine to form molecular hydrogen, which plays no significant role in diamond CVD. The recombination, or loss, of atomic hydrogen proceeds very rapidly on surfaces and comparatively slowly in the gas phase. This relationship arises because recombination of two hydrogen atoms requires a release of energy in order to proceed. In the gas phase, quantum mechanical effects prohibit the release of the recombination energy when only two hydrogen atoms collide. Thus, gas-phase recombination of atomic hydrogen requires simultaneous collision of at least three bodies, two of which are hydrogen atoms. The third body, which may be another hydrogen atom, molecule, or non-hydrogen molecule, dissipates the recombination energy as kinetic energy.
On surfaces, the required kinetic energy dissipation pathway is provided by the surface itself. The recombination of atomic hydrogen on surfaces is a two-body collision because of this, and it proceeds very rapidly compared to gas phase recombination. The recombination energy appears as heat in the substrate. Much of the substrate heating observed in diamond deposition arises from the energy of atomic hydrogen recombination.
Because the manufacture of atomic hydrogen requires substantial energy, and because the species, once made, is continuously and rapidly destroyed by recombination with itself and with other molecular species, successful diamond CVD requires manufacture of enormous amounts of atomic hydrogen compared with the amounts which actually participate in diamond deposition. This is the root of the inefficiency of current diamond CVD art.
A common feature of nearly all diamond CVD processes is the manufacture of atomic hydrogen at distances relatively remote from the deposition region. Thus, microwave diamond CVD entails the formation of microwave plasma regions, the main bulk of which do not contact the deposition surface and which may be several inches away from that surface. This provides ample opportunity for loss of valuable atomic hydrogen through recombination.
Similarly, diamond CVD practiced with heated filaments suffers from relative inefficiency of atomic hydrogen production (due to limits on filament temperature) as well as losses due to the inability to locate filaments in immediate proximity to the deposition surface, which limitation arises from substrate overheating due to radiation of energy from the filament.
A third common means of diamond CVD, use of electrically-driven plasma torches which manufacture atomic hydrogen principally through superheating a stream of molecular hydrogen, must transport manufactured atomic hydrogen in a supersonic stream from the energy application point to the deposition region, which is typically at least several inches away. Much atomic hydrogen is lost, but because the supersonic gas stream impacts the substrate at high velocity, stagnation boundary layers are thin, and the diffusive flux of atomic hydrogen to the surface is relatively large, giving high growth rates for this method. Overall efficiency, however, remains poor, because of the enormous overproduction of atomic hydrogen inherent in the process compared with the small fraction that reaches the growth surface.
Diamond CVD growth efficiency is often expressed in terms of the weight of diamond formed per unit time as a function of the amount of power supplied to the system. While this figure of merit ignores the consumption of precursor gases, it is a useful index of the energy efficiency of a diamond CVD process and makes possible an efficiency comparison among different diamond CVD processes. A more comprehensive index of diamond growth efficiency will be presented below.
As reported (MRS Bulletin, September 1998, volume 23, no. 9, pp 22-27, in particular FIG. 3 on page 24), the best diamond CVD processes have reached a power efficiency of approximately 6.25 kilowatts (kW) per hour per carat of diamond deposited. A carat is equal to 0.2 grams. As reported in this reference, this efficiency level has been reached by application of successively greater amounts of power to the growth system, reaching up to 200 kW in some systems.
Unfortunately, the capital cost of equipment capable of sustaining such extremely energetic processes is large. Thus, the modest increase in deposition efficiency gained by use of more power is offset by the resultant economic penalty. The net result of this technology development pathway has been an increase in diamond deposition rate without significant decrease in deposition cost. Breakout from the constrained high cost of diamond synthesis has been a long-sought goal in the art.
A more useful diamond CVD growth efficiency index should account for consumption of gases as well as energy use. In the same manner that energy consumption can be expressed in terms of kilowatt-hours per carat of diamond produced, hydrogen and methane consumption can be expressed in terms of standard liters per carat of diamond produced. If gas consumption in standard liters of hydrogen and methane per carat of diamond produced is added to electrical power consumption in kilowatt-hours per carat of diamond produced, and the inverse of that sum is computed, an index results that correctly reflects diamond CVD process efficiency by varying inversely with energy and gas consumption rates. This index, referred to herein as the “raw composite growth efficiency” makes it possible to compare the growth efficiencies of disparate diamond CVD processes.
Process efficiencies for the principal means of diamond CVD are shown in Table 1. The data have been compiled from literature reports and from experience with each of the listed modalities. Energy efficiency is expressed in terms of kW-Hr/carat of diamond produced, while raw material consumption is expressed as liters of gas consumed per carat of diamond produced. Raw composite growth and normalized composite growth efficiencies are calculated as stated above, and are presented with the efficiency of the arc jet modality normalized to 100%.
TABLE 1Comparative Diamond CVD Process EfficienciesCurrentArc JetHot FilamentMicrowaveInventionPower6.844.634.12.1consumption,KW-Hr/caratHydrogen2,04866954686consumption,standard liters/caratMethane102.433.427.34.3consumption,standard liters/caratRaw composite0.0004640.001340.001650.00792growthefficiencyNormalized100%289%355%1,700%compositegrowthEfficiency
The foregoing table discloses the relative growth efficiencies of diamond CVD methods, including that achieved by this invention. No diamond deposition method, other than this invention, is known to produce diamond at a raw composite growth efficiency greater than approximately 0.00165 as computed according to the method shown above. These growth efficiencies are understood to be in reference to CVD diamond material of commercially useful purity. One means of establishing purity of a CVD diamond material, and a means commonly employed in the art, is through use of Raman spectroscopy. So measured, the Raman signature of commercially useful CVD diamond shows an sp3 (diamond-bonded carbon) peak having a full width at half maximum value of 12 wavenumbers or less, and exhibits non-sp3 features of intensity less than 50% of the sp3 peak intensity in the wavenumber region from 1200 cm−1 to 1700 cm−1.